Automatic lap control based on using piezoelectric monitor blanks has been described in prior-art U.S. Pat. Nos. 4,407,094, 4,199,902, and 4,197,676. It has since been implemented and applied as a thickness measuring instrument called "Automatic Lap Controller". One of its applications is in lapping and polishing of quartz resonator blanks in planetary lap machines, where it monitors the blank frequencies via an electrode imbedded in one of the lap plates. As the blanks are lapped thinner, their resonance frequency increases. When a target frequency is reached, lapping is stopped.
The operating principle is illustrated in FIG. 1. It shows a partial and simplified cross section of a planetary lap machine with an upper lap plate (2), lower lap plate (4), carrier (6), blank (8), electrode (12), and a lap plate center axis (18). A measurement circuit (20) applies a sweep frequency signal to electrode (12) and senses the current through the electrode and the adjacent gap between upper and lower lap plates to ground. When the electrode faces a blank, the circuit measures the blank's resonance frequency.
FIG. 2 shows a top view of the lap machine with the upper lap plate removed. A lower lap plate (4) supports eight carriers (6-1 . . . 6-8) which are driven by an outer gear ring (30) and inner gear ring (32) in planetary motion around the center axis (18). Each carrier carries 5 blanks in 5 holes. Blank 8 of FIG. 1 is located in carrier 6-1, and electrode 12 is shown as being located concentrically with blank 8. As the carriers rotate, blanks are measured as they pass the electrode.
The purpose of lapping is to lap a load of blanks to a target frequency while minimizing the "spread" between the highest and lowest blank frequencies. The spread of a lap load is the difference between the highest and the lowest frequency in the load. Its exact value cannot be determined during lapping because it is not possible to measure all frequencies at the same time and because all frequencies are increasing during the process. Instead, an approximate value is defined and determined as the difference between the highest and the lowest frequency of the lap load observed during a predetermined time interval.
The Automatic Lap Controller facilitates the lapping process in two ways: by terminating it when the highest blank frequency reaches a target frequency, and by monitoring the frequency spread during lapping.
Present lap controllers have limitations in that they can only monitor the spread of the whole lap load. Typically, a load of blanks has a spread when it is loaded into the lap, and properly operating lap machine reduces this spread during lapping by equalizing the thickness of the blanks. However, there are several reasons that can prevent a spread reduction and even produce a spread increase during lapping.
One reason can be a difference in the lap performance within individual carriers. For instance, the lapping in one carrier may be erratic and produce a spread larger than in all the others. Or, the lap rate in one carrier may differ significantly from that in the others. Both cases result in a large overall spread, although the spread in the individual carriers may be small. Present methods cannot detect the reason for a large overall spread nor provide a clue for correcting the problem.
This application discloses a lap controller that can identify and monitor the values for blank frequencies and frequency spread in each individual carrier, and that can signal the termination of lapping when these values reach a predetermined target.